The present invention relates to key telephone systems and more particularly to such a system which uses four wires for transmission of voice and audio signals to and from the telephone stations which are part of the system. The system of the present invention include a plurality of telephone stations each having a two wire audio input port, a separate two wire audio output port, and a data port. Duplex communication paths are established through a novel crosspoint matrix.
Key telephone systems generally comprise a plurality of telephone stations (also referred to as "telephone sets") and a central key switch unit ("KSU") which interconnects the telephone stations with each other and to outside telephone trunk lines. The KSU provides control functions for the key telephone system and establishes communication paths between telephone stations. When outside calls are involved, the KSU establishes communication paths between telephone stations and telephone trunk lines.
Each telephone trunk line comprises, at a minimum, two wires called a wire pair connected to the telephone company central office through a "local loop". One of the wires is called T (for tip) and the other is called R (for ring). The tip and ring designations refer to the tip and ring parts of the plugs used by a telephone operator in the manual switchboards used in the past.
The two wires scheme used in the past by the telephone company for connecting a telephone line to a telephone station has been maintained in key telephone systems for commercial use. Thus, in telephone stations used with modern key telephone systems, only two wires are typically used for the transmission and reception of audio (e.g. voice) communications to and from telephone stations. An additional two wires is generally used in such systems to transmit data among the telephone stations and the KSU. Such data transmission provides the necessary system control and enables the implementation of a variety of features provided in the telephone system. Thus, each telephone station in conventional key telephone systems interfaces with the KSU using only four wires; namely, a two wire pair for the audio communication and a two wire pair for the transmission of digital data.
Although coupling to each telephone station with only two wires for audio communications (a "two-wire" system) is efficient in terms of cable utilization and expense, each telephone station must contain circuitry to (1) separate audio input from audio output signals, (2) provide local switching for various system functions (such as hands-free communication), (3) provide for line balancing and compensation, (4) provide an acceptable sound level for every telephone conversation, and (5) provide the necessary isolation between the voice transmitter and receiver portions of the telephone station. Further, at the KSU end of this system, the use of only two wires for audio communication can lead to complications in achieving desirable system features, such as call announcing and conference calling.
It would be advantageous to provide an economical key telephone system in which the transmit and receive audio communication paths are completely isolated. Such an arrangement would eliminate the need for local switching at each telephone station to implement hands-free communication. Such structure would also eliminate the need for network circuitry within each telephone station to divide conventional single telephone transmission channels into two separate channels, one for transmit and the other for receive. Further, such a system would provide for call announcing without the need for switching within the telephone station and a conference calling capability which is implemented economically and in straightforward manner.
The present invention provides such a key telephone system through the use of a two wire audio input port and a separate two wire audio output port at each telephone station (a "four-wire" system). A novel crosspoint matrix is provided as part of the present key telephone system to interconnect transmit and receive signal paths and thereby complete duplex communication channels.